The present invention relates to dimensional metrology and can be used for certification of scanning measuring devices used for measurement of sizes in micron, submicron and nanometer scales.
Scanning measuring devices, such as optical scanning microscopes which include confocal microscope, scanning optical microscope of a near field, scanning electron microscopes, as well as scanning tunneling and atomic-force microscopes which are generally designated as scanning probe microscopes are used in growing numbers of small and super small sizes in a modern industry, in particular in micro electronics as described for example in “The International Technology Road Map for semiconductors SIA Sematech, 1999”. Potentially high accuracy of measurements achievable with the use of the above listed devices is limited. The reason is in particular the presence of uncontrollable, but significant errors in their scanning systems, or in other words non-linearity of moving of a mechanical, optical or electronic probe on a surface of an object. In U.S. Pat. No. 5,825,670 it is stated that the nonlinearity of development measuring tools on a basis of scanning probe microscopes reaches several percentage points and various ways of indemnification of these errors caused by nonlinearity of scanning are considered. In “Magnification calibration of CD/SEM s for sub-100 nm metrology” by A. Sicignano, et al, presented at SPIE's 45th Annular Meeting, International Symposium on Optical Science and Technology, San Diego, Calif., August 2000, detection of significant non linear distortions with fixing of the images of micro objects, whose sizes are measured in a Scanning Electronic Microscope was reported. The methods of detection and compensation of non linear distortions disclosed in the last two references provide the use of special test-objects with a periodic structure, i.e. one-dimensional or two dimensional diffraction gratings. It is assumed that the above mentioned test objects represent the sets of quite identical elements (features): one-dimensional strips or two-dimensional figures located at strictly identical distances from each other. This idealized representation is inadequate to the reality. Even more perfect diffraction gratings created by nature itself, in particular the patterns from atoms on the surface of single mono crystals do not completely respond to such idealization for many reasons: because of mosaic character of the surface of real crystals, influence of point defects of structure-foreign atoms, vacancies, dislocations, stacking folds and other deviations from strict periodicity. In addition, the interatomic distances in crystals are too small to be used as a scale for measuring tools on the basis of the above mentioned microscopes: in the microscope of the above mentioned types the architecture of atomic pattern is simply indiscernible. This realization corresponds even less to hand made test objects because of the “technological noises” which accompany any process of manufacture of any periodic structure.
It is best to be admitted to the imperiodic test object to a certain degree is imperfect: the elements of diffraction gratings are not identical and they are not located at equal distances, which causes a non uniformity of pitch of the diffraction gratings in different places.
It is therefore possible to conclude the nonlinearity of the image in scanning system which is determined in experiments with diffraction gratings is actually an imaginary value. It is obvious that the imaginary nonlinearity of the image as a result of interaction of two different components: A—actual nonlinearity of scanning devices; and B—non uniformity of a pitch of the test objects. It is desirable to eliminate or to minimize the contribution of the actual nonlinearity of the scans.
The non uniformity of the pitch of the diffraction grading is an objective and important characteristic of the test object, and its compensation has an insulated issue or within the imaginary nonlinearity of the image is unacceptable, since it represents actually a deliberate distortion of the image of a selected portion of the test object. When real, non perfect diffraction gratings are utilized, a paradoxical situation is created: The more accurate is the compensation of the imaginary nonlinearity of image, the greater is “over compensation” of the real nonlinearity of the scan. It can be seen that if the non uniformity of the test object exceeds the real nonlinearity of scan, the compensation of the imaginary nonlinearity of the image in accordance with the above mentioned U.S. patent will lead to worsening of the actual nonlinearity of the system, instead of its improvement. It is therefore important to determine the contribution of the nonlinearity of scan and non uniformity of the test object into a joint, imaginary nonlinearity of the image.